The invention relates to aerial work platforms and, more particularly, to a hydraulic drive system for an aerial work platform.
Aerial Work Platforms (AWPs) typically use hydrostatic drives systems. They could be two- or four-wheel drive with a gear reduction and a hydraulic drive motor at each driven wheel. A hydraulic pump is typically driven directly from the engine and includes a network of traction valves or flow dividers to split the flow to each driven wheel. These flow dividers are intended to divide the total flow among each driven wheel even if one wheel would lose traction. The flow dividers typically include provisions for leakage or bypass of perfect flow division to allow for varying wheel speeds, as would be required for steering utilizing the Ackerman principle. Otherwise, wheels could be forced to slide during turns or the engine could stall.
If this bypass is too large, the flow division is compromised as a large portion of the flow could be lost to leakage. This situation is also seen when the flow dividers are sized for high flow conditions and used for the low flow conditions as the resulting leakage rate of the dividers can be a much larger portion of the available flow. This is of particular importance with AWPs since much of the driving is done at low flow conditions and with a migrating center of mass potentially unloading the ground contact load of the off side tires.
AWPs traditionally are designed with these traction valves sized for the high flow rate needed to achieve a max drive speed, which influences the efficiency and power requirements of not only high flow conditions but also low flow performance.